Add customization to signtool parallel repack limits

[mmitche]

• This issue is blocking
• This issue is causing unreasonable pain

Right now roslyn is sometimes hitting OOMs when repacking in parallel. I think this is because they have a ton of packages that are right around 65MB and we are probably trying to repack them all at the same time. The current limit is 2GB / 16 (128MB). Lowering the repack limit by half would still see most of those packages in below the limit. Lowering to 48MB for roslyn would probably work.

I think adding customization for this is probably the best way to go.

[mmitche]

#14243

[missymessa]

PR for this work was merged. Closing.